<!DOCTYPE html>
<html lang="zh-cn">
<head>
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>二级菜单</title>
    <style>
        li{
            list-style: none;
        }
        a{
            text-decoration: none;
            color: black;
        }
        .u2{
            display: none;
            position: fixed;
        }
        .u1>li{
            float: left;
            margin-left: 80px;
        }
        .u1>li:hover .u2{
            display: block;
        }
    </style>
</head>
<body>
    <ul class="u1">
        <li><a href="#">一</a></li>
        <li><a href="#">二</a>
            <ul class="u2">
                <li><a href="#">二二</a></li>
                <li><a href="#">二二</a></li>
                <li><a href="#">二二</a></li>
            </ul>
        </li>
        <li><a href="#">三</a></li>
    </ul>


</html>